Salix herbacea (Dwarf Willow,
Least Willow or Snowbed Willow) finds the equivalent of a sunbed
on some exposed rock. The rock provides extra warmth
which it retains from the sun so the tree (as it strictly speaking
is even though it only a few inches/cm high) spreads its leaves
over the surface to make the most of the short spring and summer
months.

Trees such as these are often decades old, probably well
over 50 years in the case of this tree with it's relatively
broad trunk.